This invention relates to magnetite particles that have low residual magnetic flux density, are hexahedral, octahedral or tetradecahedral in shape and that are used as electrophotographic magnetic toners and resin-dispersed carriers, and as black pigments for coatings. The invention also relates to a process for producing such magnetite particles and their applications.
While various electrostatic copying systems are commercially available today, dry development is performed either by a two-component process using a toner and a carrier or by a one-component process using only a toner. Magnetite is used in most toners and carriers.
Magnetic particles for use as toners and carriers are required to have various characteristics and particularly those to be used in one-component development are required to have low residual magnetic flux density in order to attenuate the magnetic agglomeration of toner particles. As for the shape of magnetic particles, they are required to be angular as exemplified by hexahedrons in order to provide an enhanced abrasive effect on a photoreceptor to form a satisfactory latent-image. Thus, it is desired to provide magnetite particles that have low residual magnetic flux density and which are angular in shape as exemplified by hexahedrons.
Magnetite particles are known to assume a spherical, hexahedral or octahedral shape depending upon the conditions of their production, particularly the alkali/Fe ratio employed for the generation of the magnetite particles. Spherical magnetite particles generally have low residual magnetic flux density but hexahedrons and polyhedrons having more faces have increased residual magnetic flux density. For example, Japanese Patent Public Disclosure No. 201509/1991 proposed hexahedral magnetite particles but they are short of achieving the required low residual magnetic flux density. Japanese Patent Public Disclosure No. 144840/1994 proposed substantially hexahedral magnetite particles having blunt, almost planar, ridgelines and they are also short of achieving the required low residual magnetic flux density.
It has also been proposed that Si, Al and other elements be added to magnetite particles such that they have specified shapes or are improved in dispersibility and heat resistance. For example, Japanese Patent Publication No. 9045/1991 and Japanese Patent Public Disclosure No. 92642/1994 proposed that Si be added in order to produce spherical magnetite particles. Japanese Patent Public Disclosure No. 286723/1993 teaches an improvement in the dispersibility and heat resistance of magnetite particles by addition of Si and/or Al during and after the formation of magnetite particles. Further, Japanese Patent Publication No. 25747/1996 proposed that Si be allowed to be localized in magnetite particles, thereby reducing their residual magnetism and high electrical resistance. Japanese Patent Publication No. 51538/1993 discloses a high-density, unfogged toner made of octahedral magnetite particles having an uneven distribution of Si. Japanese Patent Public Disclosure No. 333594/1993 proposed that Si be localized in magnetite particles to produce a toner of high resolution and density.
Phosphorus (P) compounds are extensively used in the art of magnetic iron oxides but they have been rarely used in cubic or polyhedral magnetite particles. Conventionally, phosphorus is added to magnetic iron oxides either during the preparation of .alpha.-FeOOH or by doping its surface. As an example of the first method, Japanese Patent Publication No. 25546/1964 uses phosphorus in order to retard the growth of crystal nuclei; further, Japanese Patent Public Disclosure No. 25202/1983 and Japanese Patent Publication No. 18766/1993 use phosphorus in order to provide an improved particle size distribution. As an example of the second method, U.S. Pat. No. 3,652,334 teaches the use of phosphorus as an anti-sinter agent in thermal conversion to .alpha.-Fe.sub.2 O.sub.3 and Fe.sub.3 O.sub.4. However, all of these examples intend to ensure that the magnetic iron oxides as the final product will have improved dispersibility or that they are improved in shape anisotropy by virtue of the retention of acicularity.
Phosphorus may be added during the formation of magnetite particles, as taught in Japanese Patent Publication No. 46525/1985; however, this method is intended to produce Co-containing magnetite particles of high coercivity by addition of P together with Co.
Thus, no magnetite particles have been known in the art that contain P and Al, and optionally Si, that have low magnetic residual flux density and that are hexahedral, octahedral or tetradecahedral in shape, nor has been known a process for producing such magnetite particles.